You may purchase Princess transfers even if you booked your own flights. However you must provide Princess with all your flight information at least 45 days prior to departure.
How it works is, a Princess representative meets you at the baggage carousel and escorts you to the bus. Once on the bus, the next time you see your luggage is in your cabin.
It may not arrive for several hours after sailing so be sure you have anything you may need in your carry-ons.
These transfers are very reasonable and make things much easier.
Have a great trip.

If you drive to the port, a porter will meet you at the curb and take all your luggage to the ship where crew members deliver it to the cabins. Make sure the luggage tags the cruise line sends you are on all checked luggage.
Your luggage might not arrive at your cabin until several hours after you sail so take things you will need in your carry-on; medication, cameras, jacket, cruise documents, valuables should all be with you.
If you decide to use the ships transfer, yes, there is a fee and it varies in price by port. A cruise we recently booked that leaves from Ft. Lauderdale was only $10 pp for the transfer.